Hemorheological factors play an important role in the pathogenesis of different cardiovascular diseases. Diabetes mellitus resulting in micro- and macroangiopathies is one of the major risk factors for cardiovascular diseases. In our present study the hemorheological parameters (hematocrit, plasma fibrinogen level, plasma and whole blood viscosity, red blood cell aggregation) of 30 diabetic patients (mean age: 57 +/- 12 years) and 30 healthy volunteers (mean age: 31 +/- 10 years) were examined. The fundus appearance of patients showed diabetic retinopathy; non-proliferative stage in 22 cases, preproliferative or proliferative stage in 8 cases. The measured hemorheological parameters of the examined patients were in the pathological range and were significantly higher than those of healthy volunteers (hematocrit: p < 0.01; plasma fibrinogen: p < 0.001; plasma viscosity: p < 0.01; whole blood viscosity: p < 0.001; red blood cell aggregation: p < 0.01). In our study there was not significant difference in the hemorheologic variables of patients with different stages of retinopathy. Our results propose the importance of hemorheologic factors in the development of diabetic microangiopathy.